The Instrument of Government was England's first and, if we except the Humble Petition and Advice, its only written constitution. It lasted only a little over three years, from 1653 to 1657, and was soon cast aside and all but forgotten in the rapid sequence of events which followed Cromwell's death: the restoration of the Rump, then of the Long Parliament, and finally of the King himself. Thus it is definitely an aberration so far as English constitutional development is concerned, but to an American, in whose country the principle of the written constitution and its concomitant principle of the limitation of legislative sovereignty have won out, the Instrument is of much greater interest.
In order to comprehend fully why the Instrument was made and what it was designed to achieve, the role of the New Model Army and its relation to Parliament must be considered. This army had become a quasi-political body as far back as the summer of 1647 when it first assumed the role of champion of political reform. In its Declaration of the Army it declared that it was not “a mere mercenary army, hired to serve an arbitrary power of a State, but called forth and conjured by the several declarations of Parliament to the defense of their own and the people's just rights and liberties.” From there it had gone on to submit various proposals for the settlement of the kingdom, such as the Heads of the Proposals and the Agreement of the People.